1. Field of the Invention
Embodiments of the invention generally relate to a catalyst withdrawal apparatus and method for regulating catalyst inventory in a fluid catalytic cracking catalyst unit.
2. Description of the Related Art
FIG. 1 is a simplified schematic of a conventional fluid catalytic cracking system 130. The fluid catalytic cracking system 130 generally includes a fluid catalytic cracking (FCC) unit 110 coupled to a catalyst injection system 100, a petroleum feed stock source 104, an exhaust system 114 and a distillation system 116. One or more catalysts from the catalyst injection system 100 and petroleum from the petroleum feed stock source 104 are delivered to the FCC unit 110. The petroleum and catalysts are reacted in the FCC unit 110 to produce a vapor that is collected and separated into various petrochemical products in the distillation system 116. The exhaust system 114 is coupled to the FCC unit 110 and is adapted to control and/or monitor the exhausted byproducts of the fluid cracking process.
The FCC unit 110 includes a regenerator 150 and a reactor 152. The reactor 152 primarily houses the catalytic cracking reaction of the petroleum feed stock and delivers the cracked product in vapor form to the distillation system 116. Spend catalyst from the cracking reaction is transfer from the reactor 152 to the regenerator 150 where the catalyst is rejuvenated by removing coke and other materials. The rejuvenated catalyst is reintroduced into the reactor 152 to continue the petroleum cracking process. By-products from the catalyst rejuvenation are exhausted from the regenerator 150 through an effluent stack of the exhaust system 114.
The FCC unit 110 includes a catalyst injection system 100 that maintains a continuous or semi continuous addition of fresh catalyst to the inventory circulating between the regenerator 150 and the reactor 152. The catalyst injection system 100 includes a main catalyst source 102 and one or more additive sources 106. The main catalyst source 102 and the additive source 106 are coupled to the FCC unit 110 by a process line 122. A fluid source, such as a blower or air compressor 108, is coupled to the process line 122 and provides pressurized fluid, such as air, that is utilized to carry the various powdered catalysts from the sources 102, 106 through the process line 122 and into the FCC unit 110.
One or more controllers 120 is/are utilized to control the amounts of catalysts and additives utilized in the FCC unit 110. Typically, different additives are provided to the FCC unit 110 to control the ratio of product types recovered in the distillation system 116 (i.e., for example, more LPG than gasoline) and to control the composition of emissions passing through the exhaust system 114, among other process control attributes. As the controller 120 is generally positioned proximate the catalyst sources 106, 102 and the FCC unit 110, the controller 120 is typically housed in an explosion-proof enclosure to prevent spark ignition of gases which may potentially exist on the exterior of the enclosure in a petroleum processing environment.
During processing, there is a dynamic balance of the total catalyst within the FCC unit. As discussed above, catalyst is periodically added utilizing a catalyst injection system. During the cracking process, some catalyst is lost through the distillation system 116, while some catalyst is lost through the effluent exiting the regenerator 150. The addition rate required to maintain a desired level of catalytic activity sometimes matches the loss rate through attrition, and in this case, there is no need for corrective measures.
If the amount of catalyst within the FCC unit diminishes over time, the performance and desired output of the FCC unit will diminish, and the FCC unit will become inoperable. Conversely, if the catalyst inventory in the FCC unit increases over time, the catalyst bed level within the regenerator reaches an upper operating limit, necessitating a small catalyst withdrawal to prevent unacceptably high catalyst emissions into the flue gas stream, or other process upsets.
Catalyst withdrawal systems are normally sized for the most demanding operating case, which is when the entire catalyst inventory is removed during a short period of time when the FCC unit is shut down. The periodic operational-type catalyst withdrawals for level control addressed by this invention can take place once a day, once a week, or once a month, depending on how quickly the catalyst inventory level is building. Because these sudden changes in regenerator bed level cause changes in the pressure balance of the FCC unit, they may result in changes in catalyst circulation rate, reactor temperature and/or regenerator temperature, and these upset the stability of the FCC unit operation. As these changes in the dynamic equilibrium force the FCC unit away from its operating limits, the desired product mix and/or effluent composition may not be obtained. As the FCC unit is a major profit center in most refineries, a great deal of time and investment made by refineries to ensure the FCC unit is always operating against its operating limits, thereby maximizing profitability. Anything that forces the operation of the FCC unit away from these limits reduces profitability and is a major detriment to the refiner. Thus, it would be highly desirable to stabilize the FCC operation by eliminating sudden catalyst withdrawals, thus maintaining the dynamic balance of catalyst in a FCC unit.
Therefore, there is a need for a catalyst withdrawal apparatus suitable for use with a fluid catalytic cracking catalyst unit.